Objective lens and optical device

ABSTRACT

An objective lens in which a first wavelength is used in reading a first optical disk, a second wavelength is used in recording or reading a second optical disk, and a phase shifter provided with annular step portions having a center coincident with an optical axis is formed in a single surface of the objective lens, the phase shifter producing a phase difference with respect to light having the first wavelength and light having the second wavelength, is presented. The objective lens can read well both a CD and a DVD.

TECHNICAL FIELD

[0001] The present invention relates to an objective lens adapted torecording or reading an optical disk such as a CD (compact disk), a DVD(digital video disk) etc., having a diffraction-limited performance, andto an optical device using such objective lens. The CD includes arecordable CD-R (compact disk recordable).

[0002] The present invention is to provide an objective lens capable ofcorrecting well both the on-axial aberration and the off-axialaberration in recording or reading a second optical disk, and an opticaldevice using such objective lens.

BACKGROUND ART

[0003] There have been proposed objective lenses for reading both a DVDcomprising a transparent substrate having a thickness of 0.6 mm and a CDcomprising a transparent substrate having a thickness of 1.2 mm(JP-A-10-255305, JP-A-11-16190, JP-A-11-2759).

[0004] In these conventional techniques, a phase shifter having aconcave portion or a convex portion, which is formed in an annularbelt-like shape around the optical axis as the center, is provided in asurface of the objective lens having an aspheric surface. The basicstructure of the objective lens is optimized so as to be capable ofreading a DVD, preferably. When a CD is to be read, the phase iscorrected by the phase shifter so that the aberration is made as smallas possible. As a result, for both DVD and CD, the correction canpreferably be achieved with respect to the on-axial aberration, inparticular, the on-axial spherical aberration.

[0005] In the conventional techniques, however, a preferred correctioncould not be achieved with respect to the off-axial coma aberration atthe time of reading a CD. Namely, since the off-axial coma aberrationwas large, the optical performance showed a large reduction when thelight source, the objective lens, etc. were inclined from the opticalaxis or shifted from the optical axis. Therefore, high accuracy wasrequired in positioning the objective lens, etc. in assembling theoptical device. Hence, productivity was poor.

[0006] Further, in the requirement of high accuracy in the determinationof the position of the objective lens, if a mechanism for moving thelens or the light source (a moving mechanism) was worn out, theobjective lens was inclined or shifted from the optical axis, wherebythere caused gradual deterioration of the optical performance with alapse of time.

[0007] Further, since the off-axial coma aberration was large, anallowable range to the positional determination of the objective lens,in particular, a deviation of the axis in autofocus driving to theobjective lens became narrow when the optical device was actuallyoperated. Therefore, there was the problem that the optical performancewas reduced.

[0008] An object of the present invention is to solve theabove-mentioned disadvantages, and to provide an, objective lens capableof correcting well both the on-axial aberration and the off-axialaberration when plural kinds of optical disks such as, for example, DVD,CD, etc. are recorded or read, and to provide an optical device usingthe objective lens.

DISCLOSURE OF THE INVENTION

[0009] In accordance with the present invention, there is provided anobjective lens having aspheric surfaces at both surfaces, used in anoptical system in which light having a first wavelength is converged tothe data recording surface of a first optical disk and reflection lightfrom the data recording surface of the first optical disk is received bya light receiving element in recording or reading the first opticaldisk, and light having a second wavelength which is different from thefirst wavelength is converged to the data recording surface of a secondoptical disk and reflection light from the data recording surface of thesecond optical disk is received by the light receiving element inrecording or reading the second optical disk, the objective lens beingcharacterized in that a phase shifter provided with annular stepportions W having a center coincident with an optical axis of the lensis formed in a single or both surfaces of the objective lens, whereinthe phase shifter has function to produce a phase difference forreducing the aberration resulted in recording or reading the firstoptical disk with respect to the light having the first wavelength, andhas function to produce a phase difference for reducing the aberrationresulted in recording or reading the second optical disk with respect tothe light having the second wavelength.

[0010] Further, according to the present invention, there is provided anoptical device in which light having a first wavelength is converged tothe data recording surface of a first optical disk via an objective lensand reflection light from the data recording surface of the firstoptical disk is received by a light receiving element in recording orreading the first optical disk, and light having a second wavelengthwhich is different from the first wavelength is converged to the datarecording surface of a second optical disk via the objective lens andreflection light from the data recording surface of the second opticaldisk is received by the light receiving element in recording or readingthe second optical disk, the optical device being characterized in thatthe objective lens is the objective lens as described just above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1: a cross-sectional view showing an embodiment of theobjective lens according to the present invention

[0012]FIG. 2: a front view of the objective lens shown in FIG. 1 whichis observed from a side of the light source

[0013]FIG. 3: an enlarged cross-sectional view around the optical axisof a second surface of the objective lens in FIG. 1

[0014]FIG. 4: a structural diagram showing an embodiment of the opticaldevice of the present invention

[0015]FIG. 5: a structural diagram of an embodiment of the opticaldevice separate from that in FIG. 4

[0016]FIG. 6: a cross-sectional view showing the objective lensaccording to Example 1

[0017]FIG. 7: an off-axial wavefront aberration characteristic diagramof an optical system for CD in Example 1

[0018]FIG. 8: an off-axial wavefront aberration characteristic diagramof an optical system for DVD in Example 1

[0019]FIG. 9: a cross-sectional view showing the objective lens inExample 2

[0020]FIG. 10: an off-axial wavefront aberration characteristic diagramof an optical system for CD in Example 2

[0021]FIG. 11: an off-axial wavefront aberration characteristic diagramof an optical system for DVD in Example 2

[0022]FIG. 12: a cross-sectional view showing the objective lens inExample 3

[0023]FIG. 13: an off-axial wavefront aberration characteristic diagramof an optical system for CD in Example 3

[0024]FIG. 14: an off-axial wavefront aberration characteristic diagramof an optical system for DVD in Example 3

[0025]FIG. 15: an off-axial wavefront aberration characteristic diagramof an optical system for CD in Example 4

[0026]FIG. 16: an off-axial wavefront aberration characteristic diagramof an optical system for DVD in Example 4

BEST MODE FOR CARRYING OUT THE INVENTION

[0027]FIG. 1 is a cross-sectional view passing the optical axis 4 of theobjective lens according to an embodiment of the present inventionwherein the surface without having step portions is a first surface (thesurface at a light source side) . FIG. 2 is a front view of theobjective lens shown in FIG. 1 when the lens is observed from a side ofoptical disk.

[0028] The objective lens of the present invention is used in an opticalsystem in which light from light sources having different wavelengthswith respect to two optical disks is converged to respective datarecording surfaces of the optical disks, and reflection light from therespective data recording surfaces is received by a light receivingelement in recording or reading the two optical disks, respectively. Theobjective lens of the present invention has aspheric surfaces in bothsurfaces in order to increase light converging accuracy. When a firstoptical disk is recorded or read, a first wavelength is used, and when asecond optical disk is recorded or read, a second wavelength is used.

[0029] Although the objective lens of the present invention has theaspheric surfaces in both surfaces as described above, it is difficultto correct sufficiently the aberration with such measures. Accordingly,a phase shifter provided with annular step portions having centerscoincident with the optical axis, is formed in a single or both surfacesof the objective lens.

[0030] In the objective lens shown in FIG. 1, annular step portions 21,22, 23, 24, 25, 26, 27, 28, 29 and 210 having centers coincident withthe optical axis are formed in the second surface (the surface at anoptical disk side) of the objective lens. Among the step portions formedin the second surface, step portions 22, 23, 24, 25, 26, 27, 28, 29 and210 have a phase shifting function to the first wavelength and thesecond wavelength, which constitute step portions W. Namely, the stepportions having a phase shifting function to the first wavelength and aphase shifting function to the second wavelength is referred to as thestep portions W.

[0031] A step portion X having a phase shifting function with respectonly to the first wavelength may be formed in a single or both surfacesof the objective lens, and a step portion Y having a phase shiftingfunction with respect only to the second wavelength may be formed in asingle or both surfaces of the objective lens, as the case requires.

[0032] Namely, it is not always necessary that all of the plurality ofstep portions formed in a single or both surfaces of the objective lensare step portions W but a part of the plurality of step portions in asingle or both surfaces of the objective lens may be used as stepportions W. In more detail, at least one in the plurality of stepportions may be the step portion X, and at least one in the plurality ofstep portions may be the step portion Y. The number of the step portionsis not limited to those shown in FIG. 1, but can be changeddiscretionary.

[0033] By shifting the phase of light having the first wavelength, theon-axial spherical aberration of the first optical disk in recording orreading, is reduced particularly. By shifting the phase of light havingthe second wavelength, the on-axial spherical aberration of the secondoptical disk in recording or reading, is reduced particularly.

[0034] In the objective lens shown in FIG. 1, a phase shifter havingannular step portions having a center coincident with an optical axis isformed in a single surface. Thus, it is preferable to form the phaseshifter provided with step portions in a single surface of the objectivelens in consideration of easiness of processing the objective lens.However, the present invention is not limited to such but phase shiftersprovided with step portions may be formed in both surfaces of theobjective lens.

[0035] In FIG. 1, φ₂₁, φ₂₂, φ₂₃, φ₂₄, φ₂₅, φ₂₆, φ₂₇, φ_(28, φ) ₂₉andφ₂₁₀ designate respectively the diameter of an inner side of the stepportions 21, 22, 23, 24, 25, 26, 27, 28, 29 and 210; φ₂₁₁ designates aneffective diameter of the second surface, and φ₁₁ designates aneffective diameter of the first surface.

[0036] The step portions W, the step portion X or the step portion Y maybe constituted by a convex portion(s) or a concave portion(s) to beformed in the objective lens surface. Any step portion is not limited tosuch configuration that the angle, in a cross-sectional view passing theoptical axis, is a right angle, but may be inclined gently. In thefollowing description, dimensions such as distance, interval, length,thickness, etc. are expressed by a unit of mm unless specificallydescribed.

[0037] In order to construct the step portions to have a phase shiftingfunction to a first wavelength λ₁ and a second wavelength λ₂, it ispreferable that in order to shift the phase of light having the firstwavelength, dimensions and shapes of the step portions of the phaseshifter are determined so that a phase difference caused by light havinga first wavelength λ₁ is in a range of from (i−0.1)λ₂ to (i+0.1)λ₂ whenit is converted into a distance, and in order to shift the phase oflight having the second wavelength, dimensions and shapes of the stepportions of the phase shifter are determined so that a phase differencecaused by light having a second wavelength λ₂ is in a range of from(j−0.1)λ₁ to (j+0.1) λ₁ when it is converted into a distance wherein jrepresents a natural number, i represents a natural number and a phasedifference multiplied by a natural number with respect to 360° is notdeemed as the phase difference. When the phase difference is expressedby a value multiplied by a wavelength, it is deemed that the phase isconverted into a distance.

[0038] In order to obtain such structure, it is preferable that the headγ of each annular step portion of the phase shifter, λ₁, λ₂, therefractive index n₁ of a material for the objective lens 3 with respectto the first wavelength and the refractive index n₂ of a material forthe objective lens 3 with respect to the second wavelength satisfy thefollowing Formula 1, more preferably, the following Formula 2.

(i−0.1)λ₂/(n ₂−1)+(j−0.1)λ₁/(n ₁−1)≦γ≦(i+0.1 )λ₂/(n ₂−1)+(j+0.1)λ₁/(n₁−1)  Formula 1

(i−0.05)λ₂/(n ₂−1)+(j−0.05)λ₁/(n ₁−1)≦γ≦(i+0.05)λ₂/(n ₂−1)+(j+0.05)λ₁/(n₁−1)  Formula 2

[0039] Here, in a case that each aspheric surface having an annularshape is provided between the annulus of a step portion and the annulusof another step portion, or in the vicinity of the effective diameterwherein such aspheric surface is referred to as an annular belt-likeaspheric surface, a head means a distance between the crossing point ofan extension from an annular belt-like aspheric surface toward theoptical axis 4 according to Formula 4 described hereinafter and theoptical axis 4, and the apex of the surface of the objective lens inwhich the annular belt-like aspheric surface is formed. For example,there is γ₂₁ (the head of the step portion 21) or γ₂₂ (the head of thestep portion 22) in FIG. 3. This distance is along the optical axis 4.

[0040] When a numerical aperture NA₁ of the objective lens in recordingor reading the first optical disk and a numerical aperture NA₂ of theobjective lens in recording or reading the second optical disk satisfyNA₁>NA₂, it is preferred that the head (e.g., γ₂₁ in FIG. 3) of the stepportion having the smallest inner diameter (21 in FIG. 1) among the stepportions satisfies the following Formula 3.

(j−0.1)λ₁/(n ₁−1)≦γ≦(j+0.1)λ₁/(n ₁−1)  Formula 3

[0041] This step portion is a step portion having function to produce aphase difference to light having the second wavelength (i.e., to shiftthe phase of light having the second wavelength), which is referred toas the step portion Y. It is necessary to correct the aberration withrespect to the second wavelength within a range of NA₂ having a limitedrange. If the head of this step portion does not satisfy Formula 3, therecording or reading of the second optical disk may not be conductedwell.

[0042] It is preferable that there is a relation that N_(W)+N_(X)+N_(Y)is from 8 to 13 among the number N_(W) of annuli of the step portions W,the number N_(X) of annuli of at least one step portion X and the numberN_(Y) of annuli of at least one step portion Y, where N_(W)represents anatural number, N_(X) is 0 (zero) or a natural number and N_(Y) is 0(zero) or a natural number.

[0043] If the number in sum is less than 8, the on-axial sphericalaberration may not be corrected well. Further, in consideration thateffective diameters of the first and second surfaces of the objectivelens are generally 5.0 mm or less, the formation of the step portionsexceeding 13 may be difficult. Preferably, the number ofN_(W)+N_(X)+N_(Y) is from 9 to 13.

[0044]FIG. 4 is a diagram showing an embodiment of the optical deviceaccording to the present invention. In FIG. 4, reference numeral 1designates a light source, numeral 2 designates an optical medium havinga reflecting function, numeral 3 designates an objective lens, numeral 5designates an auxiliary lens, numeral 6 designates a first optical disk,numeral 6 a designates a transparent substrate of the first optical disk6 (hereinbelow, referred to as the first transparent substrate), numeral6 b designates the data recording surface of the first optical disk 6(hereinbelow, referred to as the first data recording surface), numeral7 designates a second optical disk, numeral 7 a designates a transparentsubstrate of the second optical disk 7 (hereinbelow, referred to as thesecond transparent substrate), numeral 7 b designates the data recordingsurface of the second optical disk 7 (hereinbelow, referred to as thesecond data recording surface), numeral 9 designates an aperture,numeral 10 designates a light receiving element, character S₁ designatesthe distance on the optical axis from the light source 1 to the surface(first surface) at a light source side of the auxiliary lens 5, andcharacter S₂ designates the distance on the optical axis from thesurface (the second surface) at an optical disk side of the auxiliarylens 5 to the first surface of the objective lens 3.

[0045] In the optical device shown in FIG. 4, the numerical aperture NA₁of the objective lens 3 in recording or reading the first optical diskby using the first wavelength and the numerical aperture NA₂ of theobjective lens in recording or reading the second optical disk by usingthe second wavelength, satisfy NA₁>NA₂.

[0046]FIG. 5 is a diagram showing an embodiment separate from that shownin FIG. 4. In the optical device shown in FIG. 5, light from the lightsource 1 is converged to the data recording surface of the optical diskby only the objective lens 3.

[0047] In FIG. 4, light having the first wavelength emitted from thelight source 1 is successively passed through the optical medium 2, theauxiliary lens 5 and the objective lens 3 to be introduced into andconverged on the first data recording surface 6 b. In FIG. 5, since theauxiliary lens 5 is not provided, light having the first wavelength issuccessively passed through the optical medium 2 and the objective lens3 to be introduced into and converged on the second data recordingsurface 6 b.

[0048] In FIG. 4, light having the second wavelength emitted from thelight source 1 is successively passed through the optical medium 2, theauxiliary lens 5 and the objective lens 3 to be introduced into andconverged on the second data recording surface 7 b. In FIG. 5, since theauxiliary lens 5 is not provided, light having the first wavelength issuccessively passed through the optical medium 2 and the objective lens3 to be introduced into and converged on the second data recordingsurface 7 b.

[0049] The light having the first wavelength reflected by the first datarecording surface 6 b and the light having the second wavelengthreflected by the second data recording surface 7 b are returned on lightpaths on which they have propagated, and are reflected by the opticalmedium 2 to be received by the light receiving element 10.

[0050] The optical systems of the optical devices shown in FIGS. 4 and 5form a finite type optical system as a whole, respectively. Theobjective lens 3 in FIG. 4 may be an infinite type lens when theauxiliary lens is a collimeter lens. The objective lens 3 in FIG. 5 is afinite type lens. It is because light from the light source located in arange of a finite distance with respect to an optical disk is convergedto the data recording surface of the optical disk. The objective lens 3includes an objective lens designed as an infinite type which is usablealso as a finite type even in a case that a finite type lens can beused.

[0051] Further, marks representing digital signals are recorded in thefirst data recording surface 6 b and the second data recording surface 7b. When a dimension of 1 pit for each mark is several μm or less, theoptical system of the optical device of the present invention has adiffraction-limited performance in order to achieve precise recording orreading.

[0052] As the light source 1, a laser light source or the like may bementioned, for example. Assumed that the first optical disk 6 is a DVDand the second optical disk 7 is a CD, a laser light source having awavelength of 785 nm may be used for CD, and a laser light source havinga wavelength of 655 nm may be used for DVD, for example. However,wavelengths of the light sources are not limited to the above-mentioned655 nm and 785 nm.

[0053] In the optical device shown in FIG. 4, the combination of theauxiliary lens 5 and the objective lens 3 is optimized with respect tothe thickness t₁ of the first transparent substrate 6 a so that thelight having the first wavelength is well converged on the datarecording surface 6 b, and the combination of the auxiliary lens 5 andthe objective lens 3 is optimized with respect to the thickness t₂ ofthe second transparent substrate 7 a so that the light having the secondwavelength is well converged on the second data recording surface 7 b.

[0054] In other words, in the combination of the auxiliary lens 5 andthe objective lens 3 in FIG. 4, correction of the aberration is properlycarried out with respect to the first wavelength, the object-imagedistance and the thickness t₁ of the first transparent substrate 6 a,and the aberration characteristics of the optical system for recordingor reading the first optical disk 6 by using the light having the firstwavelength are optimized either in off-axial or on-axial.

[0055] Further, in the combination of the auxiliary lens 5 and theobjective lens 3 in FIG. 4, correction of the aberration is properlycarried out with respect to the second wavelength, the object-imagedistance and the thickness t₂ of the second transparent substrate 7 a,and the aberration characteristics of the optical system for recordingor reading the second optical disk 7 by using the light having thesecond wavelength are optimized either in off-axial or on-axial. Theauxiliary lens 5 is determined so that the optimum aberration can beobtained when the objective lens 3 is used in combination.

[0056] In the optical device shown in FIG. 5, the objective lens 3 isoptimized with respect to the thickness t₁ of the first transparentsubstrate 6 a so that the light having the first wavelength is wellconverged on the first data recording surface 6 b, and the objectivelens 3 is optimized with respect to the thickness t₂ of the secondtransparent substrate 7 a so that the light having the second wavelengthis well converged on the second data recording surface 7 b.

[0057] In other words, in the objective lens 3 in FIG. 5, correction ofthe aberration is properly carried out with respect to the firstwavelength, the object-image distance and the thickness t₁ of the firsttransparent substrate 6 a, and the aberration characteristics of theoptical system for recording or reading the first optical disk 6 byusing the light having the first wavelength are optimized either inoff-axial or on-axial.

[0058] Further, in the objective lens 3 in FIG. 5, correction of theaberration is properly carried out with respect to the secondwavelength, the object-image distance and the thickness t₂ of the secondtransparent substrate 7 a, and the aberration characteristics of theoptical system for recording or reading the second optical disk 7 byusing the light having the second wavelength are optimized either inoff-axial or on-axial.

[0059] Thus, the recording or reading of the first optical disk 6 andthe second optical disk 7 in which the thickness of the transparentsubstrate of an optical disk is different from the thickness of theother, are well carried out. Further, the aberration characteristics areinfluenced by not only the thickness of the transparent substrates, butalso the refractive index of the transparent substrates although thedegree of influence is slight.

[0060] In a case that the objective lens as shown in FIG. 1 is used inthe optical device shown in FIG. 4, it is preferable that a combinationof the auxiliary lens 5, an aspheric surface in the surface including anapex in a surface of the objective lens 3 and an aspheric surface in thesurface including an apex in the other surface of the objective lens 3is so determined that the on-axial spherical aberration is from 0.08 λ₁to 0.25 λ₁ in RMS value when data in the data recording surface of thefirst optical disk 6 are recorded or read and that the on-axialspherical aberration is from 0.08 λ₂ to 0.25 λ₂ in RMS value when datain the data recording surface of the second optical disk 7 are recordedor read.

[0061] When the on-axial spherical aberration is 0.08 λ₁ or more and0.08 λ₂ in RMS value, there is little possibility that the off-axialcoma aberration would not be corrected. When it is 0.25 λ₁ or less and0.25 λ₂ or less, the objective lens 3 can easily be manufactured becausecorrection of the aberration can be achieved even when the number ofstep portions is small, and an increase in the number of the stepportions can be prevented.

[0062] In a case that the objective lens as shown in FIG. 1 is used inthe optical system shown in FIG. 4, it is preferable that the opticalsystem comprising a combination of at least one selected from annularbelt-like aspheric surfaces (for instance, annular belt-like asphericsurfaces 21 a, 22 a, 23 a, 24 a, 25 a, 26 a, 27 a, 28 a, 29 a, 210 a and211 a) formed in a surface of the objective lens, an aspheric surface inthe surface including an apex in a surface of the objective lens 3,which is opposite to the surface in which the annular belt-like asphericsurfaces are formed, and the auxiliary lens 5, is so determined that theoff-axial coma aberration in an image height of 0.05 mm is 0.03 λ₁ orless in RMS value when data in the data recording surface 6 b of thefirst optical disk 6 are recorded or read by using the light having thefirst wavelength and that the off-axial coma aberration in an imageheight of 0.05 mm is 0.03 λ₂ or less in RMS value when data in the datarecording surface 7 b of the second optical disk 7 are recorded or readby using the light having the second wavelength.

[0063] When the off-axial coma aberration in an image height of 0.05 mmis 0.03 λ or less, an allowable range in an inclination or a shift ofaxis from the optical axis of the light source, the auxiliary lens, theobjective lens or the optical disk is expanded to 0.1% or more incomparison with the case that the off-axial coma aberration is more than0.03 λ. A more preferable range of the off-axial coma aberration in animage height of 0.05 mm is 0.01 λ or less. In this case, the allowablerange is expanded to 0.5% or more in comparison with the case that theoff-axial coma aberration is more than 0.01 λ.

[0064] In a case that the objective lens shown in FIG. 1 is used in theoptical device shown in FIG. 5, it is preferable that a combination ofan aspheric surface in the surface including an apex in a surface of theobjective lens and an aspheric surface in the surface including an apexin the other surface of the objective lens is so determined that theon-axial spherical aberration is from 0.08 λ₁ to 0.25 λ₁ in RMS valuewhen data in the data recording surface of the first optical disk 6 arerecorded or read and that the on-axial spherical aberration is from 0.08λ₂ to 0.25 λ₂ in RMS value when data in the data recording surface ofthe second optical disk 7 are recorded or read.

[0065] When the on-axial spherical aberration is 0.08 λ₁ or more and0.08 λ₂ or more in RMS value, there is little possibility that theoff-axial coma aberration can not be corrected. When it is 0.25 λ₁ orless and 0.25 λ₂ or less, the objective lens 3 can easily bemanufactured because correction of the aberration can be achieved evenwhen the number of the step portions is small and an increase of thenumber of the step portions can be prevented.

[0066] In a case that the objective lens as shown in FIG. 1 is used inthe optical device shown in FIG. 5, it is preferable that the opticalsystem comprising a combination of at least one of annular belt-likeaspheric surfaces (for instance, annular belt-like aspheric surfaces 21a, 22 a, 23 a, 24 a, 25 a, 26 a, 27 a, 28 a, 29 a, 210 a and 211 a inFIG. 1) formed in a surface of the objective lens 3 and an asphericsurface in the surface including an apex in a surface of the objectivelens 3, which is opposite to the surface in which the annular belt-likeaspheric surfaces are formed is so determined that the off-axial comaaberration in an image height of 0.05 mm is 0.03 λ₁ or less in RMS valuewhen data in the data recording surface 6 b of the first optical disk 6are recorded or read by using the light having the first wavelength andthat the off-axial coma aberration in an image height of 0.05 mm is 0.03λ₂ or less in RMS value when data in the data recording surface 7 b ofthe second optical disk 7 are recorded or read by using the light havingthe second wavelength.

[0067] When the off-axial coma aberration in an image height of 0.05 mmis 0.03 λ or less, an allowable range in an inclination or a shift ofaxis from the optical axis of the light source, the objective lens orthe optical disk is expanded to 0.1% or more in comparison with the casethat it is more than 0.03 λ. A more preferable range of the off-axialcoma aberration in an image height of 0.05 mm is 0.01 λ or less. In thiscase, the allowable range is expanded to 0.5% or more in comparison withthat it is more than 0.01 λ.

[0068] When the lateral magnification in the combination of theauxiliary lens 5 and the objective lens 3 which corresponds to thecombination of the light having the first wavelength and the firsttransparent substrate 6 a in the optical device shown in FIG. 4, isindicated by β₁, the lateral magnification of the objective lens 3 incombination which corresponds to the combination of the light having thefirst wavelength and the first transparent substrate 6 a in the opticaldevice shown in FIG. 5, is also indicated by β₁, the lateralmagnification in the combination of the auxiliary lens 5 and theobjective lens 3 which corresponds to the combination of the lighthaving the second wavelength and the second transparent substrate 7 a inthe optical device shown in FIG. 4, is indicated by β₂, and the lateralmagnification of the objective lens 3 in combination which correspondsto the combination of the light having the second wavelength and thesecond transparent substrate 7 a in the optical device as shown in FIG.5, is also indicated by β₂, it is preferable that both conditionsdescribed in the following Formulae (A) and (B) are satisfied.

[0069] (A) 0.05 ≦|β₁|≦0.3 and

[0070] (B) 0.05 ≦|β₂|≦0.3.

[0071] In case that 0.05≦|β₁| and 0.05≦|β₂| are not satisfied, theobject-image distance becomes too long so that miniaturization of theoptical device becomes difficult. Further in case that |β₁|≦0.3 and|β₂|≦0.3 are not satisfied, correction of the aberration becomesdifficult.

[0072] Further, in the optical device shown in FIG. 4, it is preferablethat the distance S₁ on the optical axis from the light source 1 to thesurface, at a side of the light source 1, of the auxiliary lens 5 is 8mm≦S₁<25 mm. When S₁ is less than 8 mm, correction of the aberrationbecomes difficult. On the other hand, when S₁ exceeds 25 mm,miniaturization of the optical device becomes difficult.

[0073] As an example of the optical medium 2, there are a beam splitter,half mirror, prism etc., for example. The optical medium 2 is providedas the case requires. In the optical devices shown in FIGS. 4 and 5, theoptical medium 2 may not be provided so that light from the light source1 is incident directly to the objective lens 3. Further, means forsupplying data in the data recording surface of the optical disk to thelight receiving element are not limited to those as shown in FIGS. 4 and5.

[0074] The aperture 9 functions to change a numerical aperture (NA). Thereason why the aperture 9 is provided is that when a numerical apertureused for the first optical disk 6 is different from a numerical aperturefor the second optical disk 7 in recording or reading, the numericalapertures have to be adjusted by the aperture 9. when a numericalaperture used for the first optical disk 6 is the same as a numericalaperture for the second optical disk 7, the aperture 9 is generallyunnecessary. As to the aperture 9, there is a mechanical aperture or anoptical aperture, which is, however, not limited thereto.

[0075] When NA₁>NA₂ is established between a numerical aperture NA₁ ofthe objective lens used for the first optical disk and a numericalaperture NA₂ of the objective lens used for the second optical disk, astep portion may be provided, instead of the aperture 9, in an annularbelt-like region having the center coincident with the optical axis, ina or both surfaces of the objective lens to prevent the light having thesecond wavelength from passing through to thereby stop down the light tothe numerical aperture NA₂.

[0076] In FIG. 4, the auxiliary lens 5 is constituted by a single lens.However, the auxiliary lens 5 is not limited to have such structure, andit may comprise a plurality of lenses.

[0077] In the above, description has been made as to recording orreading two kinds of optical disk. However, the present invention is notlimited thereto, and recording or reading can be conducted to three ormore kinds of optical disk wherein thicknesses of these transparentsubstrates are different from each other. Further, the optical disk usedin the present invention is not limited to DVD or CD, but may be anotherkind of optical disk.

[0078] In the optical devices shown in FIGS. 4 and 5, light having thefirst wavelength and light having the second wavelength are emitted froma single light source 1. However, the present invention is not limitedthereto, and light sources for emitting the light having the firstwavelength and the light having the second wavelength may be providedseparately.

[0079] Generally, a synthetic resin is used as materials for theauxiliary lens 5 and the objective lens 3. However, it is not alwayslimited to use the synthetic resin, and glass may be used. Further, anautofocusing driving means may be provided for the objective lens in theoptical device of the present invention.

[0080] Now, the present invention will be described with reference toExamples.

[0081] An optical device as shown in FIG. 4 was prepared on the premiseof recording or reading CD and DVD wherein DVD (t₁=0.60 mm) was used asthe first optical disk 6 and CD (t₂=1.20 mm) was used as the secondoptical disk 7.

[0082] A laser light source for emitting light having a wavelength of655 nm was used for reading or recording DVD and a laser light sourcefor emitting light having a wavelength of 785 nm was used for reading orrecording CD. As to the optical medium 2, a beam splitter made of amaterial of BK7 and having a thickness of 3.00 mm was used. Thetransparent substrate of DVD was designed to have a refractive index of1.580 with the wavelength of 655 nm. The transparent substrate of CD wasdesigned to have a refractive index of 1.573 with the wavelength of 785nm. Shape of the aspheric surfaces formed in the objective lens 3including each annular belt-like aspheric surface were determinedaccording to the following Formula 4. In Formula 4, i is 2, 4, 6, 8 or10; j is 1 or 2; h is a height from the optical axis; z_(j) is adistance from a surface contact with the apex of the j th asphericsurface to a point having a height h on the aspheric surface, and r_(j),k_(j) and a_(i,j) are coefficients of the j th surface respectively.$\begin{matrix}{z_{j} = {{\left( {1/r_{j}} \right){h^{2}/\left\lbrack {1 + \left\{ {1 - {\left( {1 + k_{j}} \right)\left( {1/r_{j}} \right)^{2}h^{2}}} \right\}^{0.5}} \right\rbrack}} + {\sum\limits_{j}{a_{i,j}h^{i}}}}} & {{Formula}\quad 4}\end{matrix}$

EXAMPLE 1 Comparative Example

[0083] The shape of the objective lens 3 was such shape as shown in FIG.6. CD and DVD are both so designed as not to correct the on-axialspherical aberration but to correct well the off-axial coma aberration.

[0084] Specifications and numerical values of the optical device and theobjective lens in Example 1 are shown in upper frames of Table 1. In theupper frames of Table 1, f₁, indicates the focal length of the objectivelens 3 at a wavelength of 655 nm, f₂ indicates the focal length of theobjective lens 3 at a wavelength of 785 nm, d₁, indicates the thicknessof the center of the objective lens 3, n₁, indicates the refractiveindex of the objective lens 3 at a wavelength of 655 nm, and n₂indicates the refractive index of the objective lens 3 at a wavelengthof 785 nm.

[0085] Coefficients of the aspheric surface in the first surface of theobjective lens in Example 1 are shown in middle frames of Table 1, andcoefficients of the aspheric surface in the second surface of theobjective lens in Example 1 are shown in lower frames of Table 1. InTables described hereinbelow, E-1 means 10⁻¹. TABLE 1 f₁ =    3.337 mmd₁ ⁼   2.25 mm f₂ =    3.356 mm n₁ ⁼   1.5405 NA₁ =    0.625 n₂ ⁼  1.5372 NA₂ =    0.525 r₁ =    1.400228 a_(2,1) ⁼ −1.2368619E−1 k₁ = −1.052361 a_(4,1) ⁼   1.0004486E−2 a_(6,1) ⁼   7.656559E−4 r₂ = −3.84781 a_(2,2) ⁼   6.0585863E−2 k₂ = −10 a_(4,2) ⁼   6.3972816E−2a_(6,2) ⁼ −1.1545647E−3

[0086] A collimeter lens was used as the auxiliary lens and the shape ofthe aspheric surface of the auxiliary lens was determined according toFormula 4. Coefficients of the aspheric surface of the auxiliary lensare shown in upper frames of Table 2. In an upper frame of Table 2,f_(c1) indicates the focal length at a wavelength of 655 nm, f_(c2)indicates the focal length at a wavelength of 785 nm, d_(c), indicatesthe thickness at the center, n_(c1), indicates the refractive index at awavelength of 655 nm, and n_(c2) indicates the refractive index at awavelength of 785 nm. In Examples 2 to 4, the auxiliary lens asindicated in the upper frames of Table 2 was also used.

[0087] Specifications of an optical system comprising the auxiliary lensas indicated in the upper frames of Table 2 and the objective lens inTable 1 are indicated in lower frames of Table 2. In a lower frame ofTable 2, P₁ indicates the distance (operating distance) between thesecond surface of the objective lens and the surface at an objectivelens side of the first optical disk 6 at a wavelength of 655 nm, and P₂indicates the distance (operating distance) between the second surfaceof the objective lens and the surface at an objective lens side of thesecond optical disk 7. TABLE 2 f_(C1) = 18.00 mm r₁ =   37.05092 f_(C2)= 18.11 mm r₂ = −12.43312 d_(C) =  2.45 mm k₁ =   19.57529 n_(C1) = 1.5405 k₂ =  −1.007722 n_(C2) =  1.5372 a_(2,1) =  −2.1713973E−3 S₁ =18.01 mm a_(2,2) =  −6.4513367E−4 S₂ =  5.25 mm a_(4,1) =   8.6229452E−5 a_(4,2) =    1.201001E−4 a_(6,1) =    0.0 a_(6,2) =   0.0 Optical system Optical system for CD for DVD Aperture (diameter)3.54 mm 4.20 mm Lateral magnification −0.184 −0.183 of optical system(β₁, β₂) P₁, P₂ 1.38 mm 1.72 mm

[0088]FIG. 7 shows the off-axial wavefront aberration characteristics ofthe optical system for CD. In FIG. 7, the solid line shows the wavefrontaberration including all kinds of aberration. The broken line shows onlythe off-axial coma aberration in the off-axial wavefront aberration.FIG. 8 shows the off-axial wavefront aberration characteristics of theoptical system for DVD. The solid line and the broken line have the samemeaning as in FIG. 7. Further, in the aberration characteristic diagramsdescribed hereinbelow, solid lines and broken liens have the samemeaning as in FIG. 7. The aberration characteristic diagram on eachExample and aberration values in Tables described hereinbelow are allbased on calculated values.

[0089] An objective lens and an auxiliary lens in Example 1 wereprepared by injection-molding a plastic material, and an optical devicein Example 1 was fabricated. When the recording or reading of DVD and CDwere performed in the optical device, accurate recording or readingcould not be performed for both DVD and CD.

EXAMPLE 2 Comparative Example

[0090] An objective lens was formed to have such shape as shown in FIG.9 wherein specifications of the lens were the same as those of Example 1as described in the upper frames of Table 1, which are basicspecifications. A phase shifter was provided in a second surface of theobjective lens in order to correct the on-axial spherical aberration forDVD. The shape of the first surface of the objective lens was the sameas the first surface of the objective lens in Example 1. In thefollowing, an aspheric surface in the second surface of the objectivelens will be described by using the same reference numerals in FIG. 9.

[0091] An aspheric surface 31 a in the surface including the apex in thesecond surface of the objective lens was formed in the same manner asthe aspheric surface in the surface including the apex in the secondsurface of the objective lens in Example 1. The off-axial comaaberration in recording or reading DVD or CD is corrected well by thefirst surface and the aspheric surface 31 a of the objective lens inFIG. 9.

[0092] The off-axial coma aberration in recording or reading DVD or CDis corrected well by the first surface and annular belt-like asphericsurfaces 32 a, 33 a, 34 a and 35 aof the objective lens in FIG. 9. Theannular belt-like aspheric surface 32 a and the annular belt-likeaspheric surface 34 a have the same head, and coefficients of theaspheric surfaces are also the same.

[0093] Coefficients of aspheric surface of the annular belt-likeaspheric surfaces 32 a, 34 a are shown in a left upper frame of Table 3,and coefficients of aspheric surface of the annular belt-like asphericsurfaces 33 a, 35 a are shown in a right upper frame of Table 3. Each ofthe heads is shown in middle frames of Table 3. Further, φ₃₁, φ₃₂, φ₃₂,φ₃₃, φ₃₄ and φ₃₅ are shown in lower frames of Table 3. Since the headsγ₃₁, γ₃₂, γ₃₃and γ₃₄ shown in upper frames of Table 4 are calculated asj=0 (zero) in Formula 1, the phase of a laser light having a wavelengthof 785 nm for CD is not shifted, and only the phase of a laser lighthaving a wavelength of 655 nm for DVD is shifted. Accordingly, accuraterecording or reading of CD can not be performed although accuraterecording or reading of DVD can be performed. TABLE 3 r₂ =  −3.8529795r₂ =  −3.853765 k₂ = −10 k₂ = −10 a_(2,2) =    6.057611E−2 a_(2,2) =   6.070106E−2 a_(4,2) =    6.394815E−3 a_(4,2) =    6.358029E−3 a_(6,2)=  −1.152933E−3 a_(6,2) =  −1.147420E−3 Head γ₃₁ of annular 1.46 μm stepportion 31 Head γ₃₂ of annular 2.92 μm step portion 32 Head γ₃₃ ofannular 1.46 μm step portion 33 Head γ₃₄ of annular 2.92 μm step portion34 φ₃₁ 0.760 mm φ₃₂ 1.152 mm φ₃₃ 2.146 mm φ₃₄ 2.984 mm φ₃₅ 3.492 mm

[0094]FIG. 10 shows the off-axial wavefront aberration characteristicsof the optical system for CD. Further, FIG. 11 shows the off-axialwavefront aberration characteristics of the optical system for DVD. Anobjective lens 3 having the shape as in Example 2 and an auxiliary lens5 were prepared by injection-molding a plastic material, and an opticaldevice in Example 3 was fabricated. When recording or reading wasconducted to DVD and CD in this optical device, accurate recording orreading could not be performed for CD although accurate recording orreading could be performed for DVD.

EXAMPLE 3 Comparative Example

[0095] An objective lens was formed to have such shape as shown in FIG.12 wherein specifications of the lens were the same as those of Example1 as described in the upper frames of Table 1, which are basicspecifications. A phase shifter was provided in a second surface of theobjective lens as shown in FIG. 12, in order to correct the on-axialspherical aberration for CD. The shape of a first surface of theobjective lens was the same as the first surface of the objective lensin Example 1. In the following, an aspheric surface in the secondsurface of the objective lens will be described by using the samereference numerals as in FIG. 12.

[0096] An aspheric surface 41 a in the surface including the apex in thesecond surface of the objective lens was formed in the same manner asthe aspheric surface in the surface including the apex in the secondsurface of the objective lens in Example 1. The combination of theaspheric surface in the first surface and the aspheric surface 41 a ofthe objective lens in this Example was not so designed as to be able tocorrect the on-axial spherical aberration for both CD and DVD, but wasso designed as to correct well the off-axial coma aberration for both CDand DVD, in the same manner as the objective lens in Example 1.

[0097] Shape of aspheric surface of annular belt-like aspheric surfaces42 a, 43 a, 44 a, 45 a and 46 a are determined in combination of theaspheric surface of the first surface and in consideration of therespective heads so that the off-axial coma aberration can be correctedwell for both CD and DVD. The shape of aspheric surface of an annularbelt-like aspheric surface 47 a was determined in combination of theaspheric surface of the first surface so that the off-axial comaaberration can be corrected well for both CD and DVD.

[0098] A step portion 41 and a step portion 45 have the same head, and astep portion 42 and a step portion 44 have the same head. The asphericsurface 41 a and the annular belt-like aspheric surface 47 a have thesame coefficient of aspheric surface (q.v. Example 1). An annularbelt-like aspheric surface 42 a and an annular belt-like asphericsurface 46 a were formed to have the same coefficient of asphericsurface. An annular belt-like aspheric surface 43 a and an annularbelt-like aspheric surface 45 a have the same coefficient of asphericsurface.

[0099] Coefficients of aspheric surface of the annular belt-likeaspheric surface 42 a and the annular belt-like aspheric surface 46 aare shown in a left upper frame of Table 4, coefficients of asphericsurface of the annular belt-like aspheric surface 43 a and the annularbelt-like aspheric surface 45 a are shown in a right upper frame ofTable 4, and the coefficient of aspheric surface of the annularbelt-like aspheric surface 44 a is shown in a left lower frame of Table4. Heads γ of annular step portions are shown in upper frames of Table5. Further, φ₄₁, φ₄₂, φ₄₃, φ₄₄, φ₄₅, φ₄₆ and φ₄₇ are shown in lowerframes of Table 5.

[0100] Since head γ₄₁, γ₄₂, γ₄₃, γ₄₄, γ₄₅ and γ₄₆ shown in upper framesof Table 5 are calculated as i=0 (zero) in Formula 1, the phase of alaser light having a wavelength of 655 nm usable for DVD is not shifted,and only the phase of a laser light having a wavelength of 785 nm for CDis shifted. Accordingly, accurate recording or reading of DVD can not beperformed although accurate recording or reading of CD can be performed.TABLE 4 r₂ =  −3.8507155 r₂ =  −3.8509334 k₂ = −10 k₂ = −10 a_(2,2) =   6.062271E−2 a_(2,2) =    6.074373E−2 a_(4,2) =    6.382655E−3 a_(4,2)=    6.343845E−3 a_(6,2) =  −1.151587E−3 a_(6,2) =  −1.145419E−3 r₂ = −3.8508415 k₂ = −10 a_(2,2) =    6.087419E−2 a_(4,2) =    6.302380E−3a_(6,2) =  −1.138902E−3

[0101] TABLE 5 Head γ₄₁ of annular 1.21 μm step portion 41 Head γ₄₂ ofannular 2.42 μm step portion 42 Head γ₄₃ of annular 3.63 μm step portion43 Head γ₄₄ of annular 2.42 μm step portion 44 Head γ₄₅ of annular 1.21μm step portion 45 Head γ₄₆ of annular 0.00 μm step portion 46 φ₄₁ 0.580mm φ₄₂ 0.976 mm φ₄₃ 1.412 mm φ₄₄ 2.266 mm φ₄₅ 2.446 mm φ₄₆ 2.546 mm φ₄₇(Effective diameter) 3.492 mm

[0102]FIG. 13 shows the off-axial wavefront aberration characteristicsof the optical system for CD. Further, FIG. 4 shows the off-axialwavefront aberration characteristics of the optical system for DVD. Anobjective lens having the shape in Example 3 and an auxiliary lens wereprepared, and an optical device in Example 3 was fabricated. Whenrecording or reading was conducted to DVD and CD in this optical device,accurate recording or reading could be performed for CD. However,accurate recording or reading of DVD could not be performed.

EXAMPLE 4 Present invention

[0103] An objective lens was formed to have such shape of lens as shownin FIG. 1 wherein specifications of the lens were the same as those ofExample 1 described in the upper frames of Table 1, which are basicspecifications. A first surface of the objective lens was formed in thesame manner as the first surface of the objective lens in Example 1.Further, heads γ of annular step portions and the head of an annularbelt-like aspheric surface 210 aare shown in upper frames of Table 6,and diameters of annular step portions are shown in left lower frames ofTable 6.

[0104] A positional relation between step portions in Example 4 and stepportions in Examples 2 and 3 is shown in right lower frames of Table 6.Namely, the lower frames of Table 6 show that the step portions inExample 4 have the same diameter as the step portions in Examples 2 and3, and the step portions in Example 4 are formed by synthesizing stepportions in Example 2 and step portions in Example 3. TABLE 6 i j Headγ₂₁ of annular 1.21 μm 0 1 step portion 21 Head γ₂₂ of annular 2.67 μm 11 step portion 22 Head γ₂₃ of annular 3.89 μm 1 2 step portion 23 Headγ₂₄ of annular 5.35 μm 2 2 step portion 24 Head γ₂₅ of annular 6.56 μm 23 step portion 25 Head γ₂₆ of annular 5.10 μm 1 3 step portion 26 Headγ₂₇ of annular 3.89 μm 1 2 step portion 27 Head γ₂₈ of annular 2.67 μm 11 step portion 218 Head γ₂₉ of annular 1.46 μm 1 0 step portion 29 Headγ₂₁₀ of annular 2.92 μm 2 0 step portion 210 Step portion Step portionhaving the same having the same diameter as the diameter as the stepportion of step portion of the objective the objective lens in Example 2lens in Example 3 φ₂₁ 0.580 mm φ₄₁ φ₂₂ 0.760 mm φ₃₁ φ₂₃ 0.976 mm φ₄₂ φ₂₄1.152 mm φ₃₂ φ₂₅ 1.412 mm φ₄₃ φ₂₆ 2.146 mm φ₃₃ φ₂₇ 2.266 mm φ₄₄ φ₂₈2.446 mm φ₄₅ φ₂₉ 2.546 mm φ₄₆ φ₂₁₀ 2.984 mm φ₃₄ φ₂₁₁ 3.492 mm

[0105] Table 7 shows shapes of aspheric surface of annular belt-likeaspheric surfaces 21 a, 22 a, 23 a, 24 a, 25 a, 26 a, 27 a, 28 a, 29 a,210 a and 211 a. TABLE 7 Annular belt-like Having the same coefficientas the aspheric surface aspheric surface in the surface 21a includingthe apex of the second surface of the objective lens in Example 1Annular belt-like Having the same coefficient as the aspheric surfaceannular belt-like aspheric surface 42a 22a of the objective lens inExample 3 Annular belt-like r₂ =  −3.8498734, aspheric surface k₂ = −1023a a_(2,2) =    6.080284E−2 a_(4,2) =    6.326211E−3 a_(6,2) = −1.142893E−3 Annular belt-like r₂ =  −3.8512034, aspheric surface k₂ =−10 24a a_(2,2) =    6.088935E−2 a_(4,2) =    6.297075E−3 a_(6,2) = −1.137975E−3 Annular belt-like r₂ =  −3.8508623, aspheric surface k₂ =−10 25a a_(2,2) =    6.105283E−2 a_(4,2) =    6.245578E−3 a_(6,2) = −1.129929E−3 Annular belt-like r₂ =  −3.8499238, aspheric surface k₂ =−10 26a a_(2,2) =    6.120625E−2 a_(4,2) =    6.199294E−3 a_(6,2) = −1.122996E−3 Annular belt-like r₂ =  −3.850121, aspheric surface k₂ =−10 27a a_(2,2) =    6.104731E−2 a_(4,2) =    6.249557E−3 a_(6,2) = −1.130896E−3 Annular belt-like Having the same coefficient as asphericsurface the annular belt-like aspheric 28a surface 24a Annular belt-likeHaving the same coefficient as aspheric surface the annular belt-likeaspheric 29a surface 23a Annular belt-like Having the same coefficientas aspheric surface the annular belt-like aspheric 210a surface 34a inExample 2 Annular belt-like Having the same coefficient as asphericsurface the annular belt-like aspheric 211a surface 35a in Example 2

[0106]FIG. 15 shows the off-axial wavefront aberration characteristicsof the optical device for CD. FIG. 16 shows the off-axial wavefrontaberration characteristics of the optical device for DVD. An objectivelens having the same shape as in Example 4 and an auxiliary lens wereprepared and an optical device in Example 4 was fabricated. Whenrecording or reading was conducted to CD and DVD in this optical device,accurate recording or reading could be performed for both CD and DVD.

[0107] Industrial Applicability

[0108] The objective lens of the present invention is provided with aphase shifter having function to produce a phase difference with respectto light having a first wavelength and light having a second wavelength,in a or both surfaces. Accordingly, when recording or reading isconducted to a first optical disk and a second optical disk, both theon-axial aberration and the off-axial aberration can be corrected well.

[0109] Accordingly, the objective lens of the present invention providesexcellent on-axial aberration and off-axial aberration even in a case ofresulting an inclination from the optical axis or a shift of axis withrespect to the light source or the objective lens with the lapse oftime. Further, the objective lens can present a highly precise opticaldevice easily, and there is little possibility that the opticalperformance decreases with the lapse of time.

[0110] The entire disclosure of Japanese Patent Application No.2000-232184 filed on Jul. 31, 2000 including specification, claims,drawings and summary is incorporated herein by reference in itsentirety.

What is claimed is:
 1. In an objective lens having aspheric surfaces atboth surfaces, used in an optical system in which light having a firstwavelength is converged to the data recording surface of a first opticaldisk and reflection light from the data recording surface of the firstoptical disk is received by a light receiving element in recording orreading the first optical disk, and light having a second wavelengthwhich is different from the first wavelength is converged to the datarecording surface of a second optical disk and reflection light from thedata recording surface of the second optical disk is received by thelight receiving element in recording or reading the second optical disk,the objective lens being characterized in that: a phase shifter providedwith annular step portions W having a center coincident with an opticalaxis of the lens is formed in a single or both surfaces of the objectivelens, wherein the phase shifter has function to produce a phasedifference for reducing the aberration resulted in recording or readingthe first optical disk with respect to the light having the firstwavelength, and has function to produce a phase difference for reducingthe aberration resulted in recording or reading the second optical diskwith respect to the light having the second wavelength.
 2. The objectivelens according to claim 1, wherein a step portion X having function toproduce a phase difference with respect only to the light having thefirst wavelength is formed in a single or both surfaces of the objectivelens, the step portion X being an annular step portion X having thecenter coincident with the optical axis.
 3. The objective lens accordingto claim 1, wherein a step portion Y having function to produce a phasedifference with respect only to the light having the second wavelengthis formed in a single or both surfaces of the objective lens, the stepportion Y being an annular step portion Y having the center coincidentwith the optical axis.
 4. The objective lens according to claim 1,wherein convex portions or concave portions are formed in a single orboth surfaces of the objective lens to constitute a part of or the wholeof the step portions W.
 5. The objective lens according to claim 1,wherein dimensions and a shape of the step portions W of the phaseshifter are determined so that the phase difference caused by lighthaving a first wavelength λ₁ is from (i−0.1)λ₂ to (i+0.1)λ₂ when thephase difference is converted into a distance, and the phase differencecaused by light having a second wavelength λ₂ is from (j−0.1)λ₁ to(j+0.1)λ₁ when the phase difference is converted into a distance, wherei represents a natural number and j represents a natural number.
 6. Theobjective lens according to claim 1, wherein the relation of8≦N_(W)+N_(X)+N_(Y)≦13 is established among the number N_(W)of annuli inthe step portions W, the number N_(X) of annuli in a step portion X andthe number N_(Y) of annuli in a step portion Y, where N_(W)represents anatural number, N_(X) represents 0 (zero) or a natural number and N_(Y)represents 0 (zero) or a natural number.
 7. The objective lens accordingto claim 1, wherein the objective lens is used in an optical system inwhich light having a first wavelength λ₁ and light having a secondwavelength λ₂ are both converged to the data recording surface of anoptical disk via an auxiliary lens and an objective lens, the objectivelens being characterized in that: a combination of the auxiliary lens,an aspheric surface in the surface including an apex in a surface of theobjective lens and an aspheric surface in the surface including an apexin the other surface of the objective lens is determined so that theon-axial spherical aberration is from 0.08 λ₁ to 0.25 λ₁ in RMS value ina case that data in the data recording surface of the first optical diskare recorded in or read, and that the on-axial spherical aberration isfrom 0.08 λ₂ to 0.25 λ₂ in RMS value in a case that data in the datarecording surface of the second optical disk are recorded in or read. 8.The objective lens according to claim 7, wherein the auxiliary lens is acollimeter lens.
 9. The objective lens according to claim 1, wherein theobjective lens is used in an optical system in which light having afirst wavelength and light having a second wavelength are both convergedto the data recording surface of an optical disk via an objective lenswithout being passed through an auxiliary lens, the objective lens beingcharacterized in that: a combination of an aspheric surface in thesurface including an apex in a surface of the objective lens and anaspheric surface in the surface including an apex in the other surfaceof the objective lens is determined so that the on-axial sphericalaberration is from 0.08 λ₁ to 0.25 λ₁ in RMS value in a case that datain the data recording surface of the first optical disk are recorded inor read, and that the on-axial spherical aberration is from 0.08 λ₂ to0.25 λ₂ in RMS value in a case that data in the data recording surfaceof the second optical disk are recorded in or read.
 10. In an opticaldevice in which light having a first wavelength is converged to the datarecording surface of a first optical disk via an objective lens andreflection light from the data recording surface of the first opticaldisk is received by a light receiving element in recording or readingthe first optical disk, and light having a second wavelength which isdifferent from the first wavelength is converged to the data recordingsurface of a second optical disk via the objective lens and reflectionlight from the data recording surface of the second optical disk isreceived by the light receiving element in recording or reading thesecond optical disk, the optical device being characterized in that: theobjective lens is an objective lens described in claim 1.